Space scientists just added two more worlds to the Habitable Exoplanets Catalog—the online database ranking the best picks for life in the galaxy.

55 planets now make the list—all places where extraterrestrials may exist.

Or perhaps not; maybe nothing lives on any of them, not even a microbe.

“There are still so many unknowns,” says Abel Mendez, the catalog’s chief curator.

The two new planets, like their 53 predecessors, are little more than shadows to us, literally and figuratively—cosmic conundrums residing trillions of miles from our solar system.

But this much is known: Both worlds are near the size of Earth. Both might have Earth-like temperatures. Both orbit their stars in the habitable zone—the sweet spot in a system, a location where liquid water may flow and life may flourish.

Mendez, a planetary astrobiologist and professor at the University of Puerto Rico at Arecibo, says that’s enough to consider them “potentially habitable,” at least for now.

“It probably has some temperatures similar to Earth,” says Mendez. “But that’s a guess.”

And that’s also the rub.

Credit: NASA / Walt Feimer

Artist’s impression of a red dwarf star.

Credit: NASA

The habitable zone.

Kepler-1652b is so far away—about five quadrillion miles—there’s little more to do than speculate and estimate. The world is too remote for even NASA’s James Webb Space Telescope, now slated for a 2021 launch.

“It will be a long time before we know much about this planet,” sighs Mendez. “It’s screaming, ‘Learn more about me.’ But the distance makes that very hard. It may be decades before we know more. Maybe centuries.”

The other planet—HD 283869b—orbits a “K star” in the constellation Taurus, 155 light years away, approximately 900 trillion miles.